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Leah M. Akins, DCC 1

Unit 2: Introduction toOperational Amplifiers

by Leah M. Akinsbased on Operational

Amplifiers and LinearIntegrated Circuits by

Coughlin & Driscoll


Analog and Digital Systems The first op-amps were used for

performing operations in an analogenvironment:èaddition and subtractionèmultiplication and divisionèintegration and differentiation.

These operations are now performed withgreater speed, accuracy, and versatilitywithin a digital environment.

Did this do the op-amp in?


Current Op-AmpApplications

“As more digital systems are created fordata acquisition and process control,more interface circuits using op-ampsand other linear integrated circuits arealso required.” C&D, p. 2.

Op-Amp Applicationsèsignal conditioningèA/D and D/A conversionèfiltersèoscillators …. And much more.


Circuit Symbol andTerminals

-

741

+

InvertingInput, V-

Non-InvertingInput, V+

PositiveSupply, VCC

NegativeSupply,VEE

Output,Vo

Pin 3

Pin 2

Pin 7

Pin 4

Pin 6


Internal Op-Amp Circuitry(see figure 1-3 or handout)

Input Stage - Differential AmplifierèIts characteristics are very high input

impedance and large voltage gain.

Intermediate Stage - Level ShifterèOutput of the differential amplifier is directly

coupled to the intermediate stage. Itsfunction is to pass the signal unaltered whileshifting its dc voltage level to a value requiredto bias the output stage.


Internal Op-Amp Circuitry(cont.)

Output Stage - Push-Pull AmplifierèA pnp-npn push-pull amplifier configuration s

used as the output stage. This gives the op-amp its characteristic low output impedance.

Packaging and PinoutsèPackaging: see figure 1-4èPinouts: see figure 1-5

Breadboarding Suggestions : read 1-6.2


Op-Amp Terminals(see figure 2-1 or handout)

power supply terminals: +V and -V output terminal: Vo

input terminals: V+ and V-

input resistance: Rin

output resistance: Ro

input bias currents: I+ and I- differential voltage: Ed = V+ - V-


Open Loop Voltage Gain

Aol, the open loop voltage gain, isthe voltage gain supplied whenthere is no negative feedback fromoutput back to input.

General Amplifier Equation: Vo = AvVin

Operational Amplifier Equation:Vo = Aol Ed


Typical vs. Ideal Values The table below compares some

typical values for an operationalamplifier along with the ideal values.

Description Typical Value Ideal ValueOpen loop gain 200,000 Infinite

Input resistance 2 MΩΩ Infinite, opencircuit

Input biascurrents

80 nA 0 A

Input offsetvoltage

1 mV 0 V


Significance of Real versusIdeal Value

The input offset voltage, Vos, is acharacteristic of the real op-amp.

Input offset does not occur in theideal device because thedifferential amplifier in the inputstage is made from perfectlymatched transistors.

Any mismatch in the differentialamplifier results in an input offset.


Effect of Offset Voltage

Consider that both inputs are equal, sothat Ed=0.èIdeally, Vo = Aol Ed = 0.èIn reality, the input offset voltage is also

amplified so thatVo = Aol Vos = 200,000 (1mV) = 200V !

Even with no input, the output is driven toits highest possible value as limited by thepower supply of the circuit.


The Limits

The output voltage can never exceed thepower supply voltages. See fig. 2-3.èIn a real op amp, the output is usually limited

to a value slightly less than the power supplyvoltage due to internal voltage drops. Seeinternal circuitry.èSaturation voltages are specified on the

manufacturer’s data sheet.èFor a 741 Op-Amp using +/- 15V supplies:

» +Vsat =14V and -Vsat = -13V


Open Loop Applications

Zero-Crossing Detector - fig. 2-4 Positive Voltage Level Detector - fig. 2-5 Negative Voltage Level Detector - fig. 2-6 Typical Applications of Voltage-Level

Detectorsèsee sections 2-5, 2-7, and 2-8èdiscuss


The End Readèchapter 3, C & D

HWèConsider figures 2-4, 2-5, and 2-6 and how

you would implement them on Multisim.

Lab ActivityèTest for existence of offset.èConstruct variety of op amp level detectors.

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